The present invention relates to systems for controlling the application of power to alternating current electric motors; and in particular to such devices which regulate the application of the electric power to stop the motor.
A conventional motor controller has thyristors which connect motor stator windings to alternating current supply lines. For a three-phase motor, each AC phase line usually is coupled to a separate winding within the motor by a thyristor switch formed by either a triac or a pair of inversely connected silicon controller rectifiers (SCR's). A circuit within the controller determines the proper time to trigger the thyristor switches during each half cycle of the supply line voltage. The thyristor switches are triggered in sequence as determined by the phase relationship of the voltage on each supply line. The sequence is circular in that after each iteration of triggering all of the thyristor switches, the process repeats by re-triggering them in the same order. Once a thyristor switch is triggered it remains in a conductive state until the alternating current flowing therethrough makes a zero crossing at which time it must be retriggered to remain conductive. By regulating the trigger times of the switches with respect to the zero current crossings, the intervals during which they are conductive can be varied to control the amount of voltage applied to the motor.
To start the motor, conventional motor controllers vary the thyristor switch trigger times to provide a gradual increase in the voltage. In doing so, the switches are initially triggered relatively late in each voltage half-cycle so that they are conductive for only a short period. The trigger times then become progressively earlier in each half-cycle to render the thyristor switches conductive for longer intervals and apply greater amounts of voltage to the motor until it reaches full speed.
These motor controllers often did not provide a mechanism for braking the motor when it was stopped. In response to an operator input to stop the motor, the basic controller simply disconnected the electricity allowing the motor to coast to a stop, slowed only by friction. If the motor is coupled to a mechanical load with considerable inertia, the motor and the load will continue to move for some time after the power is shut off. In many industrial applications of motors, it is important for convenience and efficient use of the driven equipment to stop this continued movement as fast as possible. Merely allowing the motor to coast to a stop was unsatisfactory. Heretofore, a mechanical brake often was coupled to the equipment and engaged when the power was turned off.
As an alternative, a direct current was sometimes applied to the stator windings of an alternating current motor to provide a braking action. In order to electrically brake an alternating current motor, it is necessary to generate a torque in the direction opposite to the direction of the the rotation of the rotor, referred to herein as "negative motor torque". In the direct current injection method of the prior art, the torque is produced by the rotor attempting to rotate in the presence of a steady magnetic field produced by the direct current applied to the stator winding. The rotating direction of the rotor's magnetization leads the direction of the magnetic field produced by the direct current through stator winding. The tendency of the rotor magnetization to align itself with the stator's magnetic field creates an alignment torque which produces a braking effect on the rotor. As is well-known, this torque is equal to the product of the stator magnetic field strength and the rotor magnetization together with the sine of the angle therebetween. More recently a stopping maneuver has been incorporated into motor controllers to create a negative motor torque by applying electricity from an alternating supply to the motor at the proper instants. After the electric current supply to motor is disconnected, the rotor magnetization starts to decay over a period of time on the order of a second or two. During this period, the rotor and its associated magnetization, rotate with respect to the stator and induce a voltage across the stator windings referred to as "back emf voltage". This voltage varies sinusoidally in time and passes through zero at the instants when the rotor magnetization is aligned with the axis of the corresponding winding. Therefore, observation of the back emf voltage, induced in the stator windings in the absence of a stator current, indicates the orientation of the rotor's magnetization.
The back emf voltage also indicates the angle between the rotor magnetization and the direction of the stator's magnetic field, if current was applied to the stator coils. Therefore, the instant to apply current pulses to the stator winding to produce a braking torque can be determined from the back emf voltage waveform across the stator windings. Specifically, a braking effect can be produced if electric current pulses are passed through the stator windings at times when the direction of the rotor magnetization is leading the direction of the magnetic field which will be produced by the stator current. The alignment torque produced by the application of the alternating current is then in a direction opposite to the rotor's rotation thereby exerting a braking torque.
Previously three-phase motor controllers sensed the back emf voltage across one stator winding of the motor. When the sensed back emf voltage and the supply voltage between the phases supplying the other two stator windings are of opposite polarity, the electricity is applied to the other two stator windings. The previous stopping maneuver applied the electric current to the same set of stator windings for a given period of time sufficient to stop the motor. U.S. Pat. No. 4,833,386 describes this technique in detail.
As the motor slowed to approximately ten percent of its full running speed, sending current through the other two windings occasionally produced a positive motor torque. Such a positive motor torque briefly accelerated the rotor of the motor prolonging the stopping time slightly. Nevertheless, the maneuver had the net effect of bringing the motor to a faster stop than merely disconnecting the electricity.
In an attempt to avoid producing an acceleration during braking, the present inventors applied the braking current through the same winding used to sense the back emf and one of the other motor windings. As with the previous technique, the current was applied in response to the sensed back emf voltage having the opposite polarity to the voltage between the supply phases for the two windings to which current is to be applied. Although the latter technique uniformly produced negative motor torque at low speed, it produced an occasional burst of positive motor torque at high speed, e.g. 90 percent of full speed.